A developing procedure of an electrostatic charge image is a procedure of attracting fine charged particles by electrostatic attraction to allow them to adhere to a surface of a support for the electrostatic charge image, and visualizing it. As a developer, a toner comprising a binder resin and a colorant contained therein is used. In some cases, the visualized image is transferred to the support as it is and fixed. Usually, however, the image is transferred to another support such as transfer paper, followed by fixing. Fixing systems include non-contact heat fixing systems such as oven fixing and contact heat fixing systems such as heat roller fixing. In general, the contact heat fixing systems are superior to the other because of their high thermal efficiency and high-speed fixing. Use of a heat roller in the contact heat fixing system can reduce power consumption, because a heat source of relatively low temperature can be used, which makes it possible to miniaturize copying machines and to save energy. Further, there is no danger of firing even when paper jamming takes place in a fixing device.
The heat roller fixing system has various advantages as described above, but has a serious disadvantage that offset is liable to occur. The term "offset" means a phenomenon that a toner constituting an image is partly transferred to a surface of a heat roller in fixing and transferred to transfer paper subsequently supplied, to thereby stain the image. As methods for preventing this phenomenon, there are proposed a method of conducting fixing while applying a surface lubricating oil such as a silicone oil to a surface of a heat roller, and a method of adding a wax represented by a polyolefin to a toner itself to impart the offset preventing property as described in JP-A-3-122664 (the term "JP-A" as used herein means an "unexamined published Japanese patent application").
For the latter method, a silicone oil coating mechanism or the like are not required, so that the structure of a fixing device becomes simple, and maintenance such as supply of the silicone oil is unnecessary. However, the wax added to the toner deteriorates the transparency of the image after fixing, because of its insufficient compatibility with a binder resin. Deterioration of the transparency does not directly lead to a problem for a monochrome copy, but arises a problem when a full color image is formed in which importance is attached to the transparency of light. It should be therefore avoided to compound the above-described wax as far as possible.
Then, the development of full color toners used in the former method, namely a oil supply type heat roller fixing method, has been desired.
Recently, in printers, facsimiles and copying machines, the demand for color copies having a large amount of information has been increased, and the image quality thereof has also been improved year by year. As a method for forming a color copy, the method described in U.S. Pat. No. 2,962,374 is generally used in which at least three electrostatic latent images are formed based on a three-color synthesis system such as a subtractive color process, and then developed with different toners of at least three colors to synthesize an image on copying paper. The color toners used in this method require development and maintenance of suitable colors, as well as mechanical, environmental and electrical stabilities to external factors such as shock, humidity and temperature. It is therefore required to optimize the design of binder resins of the toners, the kind and amount of colorants, and methods for dispersing the colorants.
As the colorants of the color toners, colorants of the organic pigment family are superior to colorants of the dye family from the viewpoints of light resistance, transfer resistance and safety. However, commercially available organic pigments in the dried powder state have the problem that it is difficult to disperse them in the binder resins. If primary particles of the pigment exist as aggregates in the toners, the light permeability which is the most important characteristic in the full color images is significantly deteriorated, resulting in a substantial deterioration of color reproduction. It becomes therefore difficult to apply them to a film for overhead projectors (OHPs).
As methods for overcoming the above-described disadvantage of the organic pigments, methods are proposed in which a commercially available organic solvent is mixed with a binder resin of a toner by heat melting at a concentration higher than a concentration of a pigment used in usual toners to produce a processed pigment, and then, the above-described processed pigment is diluted with the binder resin so as to give an optimum pigment concentration as the toner, followed by melt-mixing, thereby reducing dispersed unit of the organic pigment in the toner, as described in JP-A-62-280755, JP-A-2-293867, JP-A-3-107869, JP-A-3-107870, JP-A-3-115568, JP-A-3-185459, JP-A-3-269542 and JP-A-3-269543.
However, the commercially available organic pigments could not avoid the aggregation of the primary particles of the organic pigment at the drying stage in the production of the pigment, and could not be finely dispersed to a degree that the light permeability of the resulting toner was not affected.
Then, as an alternate method for reducing the dispersed unit of the organic pigment in the toner, JP-A-62-127847 proposes a method in which an organic pigment such as phthalocyanine deposited in a solution at the stage of pigment process is washed and concentrated, followed by heat mixing with an organic solvent in which a resin is dissolved, water and the organic solvent are removed to prepare a resin composition containing the organic pigment finely dispersed therein at a relatively high concentration (hereinafter referred to as a "flushing pigment"), and the flushing pigment is melt-kneaded with a binder resin, followed by pulverization to obtain the toner. The toner obtained by this method is fairly good in light permeability and color reproduction, compared with the above-described processed pigments.
However, when the pigment is pigment-processed, for example, from phthalocyanine in this method, phthalocyanine prepared by an acid paste process or an acid slurry process is used. Accordingly, .alpha.-form phthalocyanine crystals are formed, and .beta.-form phthalocyanine crystals excellent in color reproduction can not be obtained. Further, the above-described method can not be applied to azo lake magenta pigments or yellow pigments synthesized by an azo coupling process. Moreover, use of the organic solvent for replacing water present in hydrous paste necessitates recovery of the organic solvent. In the acid paste process or the acid slurry process, use of concentrated sulfuric acid causes the possibility of remaining of sulfuric acid in the pigment even after washing, and when a polyester is used as a binder resin, ester linkages of the polyester are disadvantageously hydrolyzed to change the characteristics of the toner.
In order to solve these problems, the present inventors previously proposed a full color toner substantially improved in light permeability and color reproduction by controlling a dispersed unit of a pigment (JP-A-4-242752). A method for producing this toner comprises the steps of heating hydrous paste of an organic pigment together with a binder resin, heat-mixing them in a pressure kneader at a temperature of 100.degree. C. or more to remove water by replacement with the molten resin, thus preparing a resin containing the organic pigment dispersed therein at a high concentration (hereinafter referred to as a "melt flushing pigment"), then melt-kneading with the binder resin, and pulverizing the resulting mixture.
The hydrous paste of the phthalocyanine pigment can be produced by the salt milling process in which water, a salt and a pigment are pulverized in a pulverizing mill applying high shear stress to obtain finely divided pigment particles. Further, hydrous paste of an azo lake magenta pigment prepared by lake formation from an azo dye can be produced by preparing a pigment dispersion in the coexistence of a rosin derivative to prevent the pigment from aggregating in lake formation, and concentrating the dispersion.
The adoption of the above-described method for producing the melt flushing pigment makes it possible to control the dispersed unit of the pigment into the toner and to improve the color reproduction and the light permeability. By the way, in the case of the full color toner, a polymer having a relatively low molecular weight and a narrow molecular weight distribution, for example, a polyester resin having a number average molecular weight of 3,200, a weight average molecular weight of 9,800 and a glass transition point of 66.degree. C., is used in order to ensure high luster, high color development and good fixing property at low temperature. As a result, use of the melt flushing pigment arises the problem that the toner rapidly melts in excess at a temperature exceeding a certain temperature to cause adhesion to a fixing roll or impregnation to paper, resulting in a reduction in image quality.
To solve the above described problems, a binder resin having a higher molecular weight might be used as long as the above described high luster, high color development and good fixing property at low temperature are not impaired. However, such a method is disadvantageous in that the molecular weight of the binder resin in the resulting melt flushing pigment is fluctuated, because the binder resin is shear-mixed with a high concentration of organic pigment in melt-flushing stage, to thereby the physical properties of the resulting toner are fluctuated. As a result, the production stability is impaired.
It has been therefore desired to develop binder resins suitable for the production of the melt flushing pigments.
As a binder resin of a toner, vinyl polymers have previously been widely used, and use of polymers having a high molecular weight has been proposed to obtain the non-offset property. However, the temperature of heat rollers is required to be established high to obtain fixed images having excellent luster, because of the high softening point of the vinyl polymer having a high molecular weight. This results in reverse movement against energy saving. Further, the toner using a vinyl polymer is liable to be attacked with a plasticizer contained in a vinyl chloride resin, and the toner itself is plasticized by contact with the plasticizer to become tacky. As a result, the plasticized vinyl chloride resin products are contaminated. That is, there is a problem with regard to the vinyl chloride resistance characteristic.
In contrast, polyester resins are excellent in vinyl chloride resistance characteristic, and low molecular weight polyester resins can be relatively easily produced. Further, the polyester resins also have the advantage that they are good in wetting to a support such as transfer paper when melted, compared with the toner in which a vinyl polymer is compounded as a binder resin, so that sufficient fixing can be conducted at a lower temperature, compared with the vinyl polymer having an approximately similar softening point. The polyester resins have therefore been come into use as a binder resin for a toner.
In JP-A-4-242752 described above, also in order to ensure the production stability in shear mixing at the melt flushing stage, a polyester obtained from terephthalic acid/bisphenol A-ethylene oxide addition product/cyclohexanedimethanol is used as the binder resin, said polyester having, for example, a number average molecular weight of 3,000 to 3,600, a weight average molecular weight of 8,700 to 9,500, a softening point of 100.degree. to 125.degree. C. and a glass transition point of 55.degree. to 68.degree. C. The polyester resin independently containing the bisphenol A-ethylene oxide addition product has a high strength, so that the above-described production stability is excellent and the fixing strength is also excellent. However, the polyester resin is very poor in pulverizing property in the toner production, and therefore, it has the problem that the production efficiency is low, resulting in too high toner cost to be practically used.
In general, the production cost of the toners produced through the respective stages of melt-kneading, pulverization and classification is mostly occupied by the cost of the pulverization and classification stages. The pulverizing property of the binder resins is therefore a very important factor in the toner production. In digital copying machines for which the future demand is promised, the tendency of toners to be reduced in particle size is increased, and at present, it is necessary to pulverize the toners to 7 .mu.m or less. The pulverizing property of the binder resins is therefore an important factor.
The toners in which the above-described polyester are used as the binder resins can be pulverized with jet pulverizers, etc., but a large amount of energy is required. In addition, the resulting pulverized products are strong in aggregation property, and poor in fluidity as powders, resulting in poor classification efficiency. Such toners strong in aggregation property are difficult to be uniformly mixed with external additives, and the external additives can not uniformly adhere to surfaces of the toners. Further, adhesion and fusion of the toners to pipes of the pulverizers, inner walls of pulverizing chambers, collision plates, etc. markedly take place, which causes a reduction in toner productivity.
Of the polyester-containing binder resins, therefore, it has been desired to develop resins suitable for the production of the melt flushing pigments and excellent in toner productivity.
On the other hand, in the full color toners, a black toner is generally used in combination with cyan, magenta and yellow toners to obtain clear black color in color images. Previously, carbon black has been commonly used as a coloring material in monochrome toners.
As a use example of carbon black in the monochrome toner, JP-A-59-218463 discloses that carbon black is added in an amount of 15% by weight or more based on the total weight of toner for giving a high-density black color and improving environmental charging stability, transferring property and development life. However, this toner can not faithfully reproduce colors from a low-density region to a high-density region, that is, from a light gray color to a deep black color.
Further, JP-A-60-129756 discloses that carbon black is added in an amount of 0.1 to 2.0 parts by weight based on 100 parts by weight of binder resin for providing a color toner having a good frictional charging property. For the toner containing such an amount of carbon black, the amount of toner developed is required to be increased to reproduce a high-density black color. As a result, problems such as fog to non-image portions and scattering of the toner to the periphery of a developing machine are encountered. Moreover, the amount of carbon black within this range gives wholly reddish-brown copies.
It has been therefore desired to develop black toners for forming full color images excellent in the above-described black color reproduction, productivity, etc.
On the other hand, with the spread of digital full color copying machines, the stability in image quality of copies, higher image quality and higher reproduction have been desired.
For realizing high image quality and high reproduction of color copies, use of small-sized color toners is proposed in JP-A-4-337738 and JP-A-6-75430. However, it is described therein that development, transfer and cleaning characteristics are deteriorated as the size of the toners is decreased, and that the stability and maintenance of image quality of copies are poor. Further, it is also described that the powder fluidity is deteriorated by a reduction in particle size of the toners, and that the frictional charging property with carriers and the environmental charging stability are also deteriorated.
As countermeasures for solving these problems, external additives externally added to a toner are studied. The present inventors previously proposed a method of externally adding fine spherical inorganic or organic particles having a particle size of 20 to 80 nm in JP-A-4-337738, and a method of externally adding titanium oxide particles having a particle size of 0.01 to 0.2 .mu.m in JP-A-6-75430.
However, even by these methods, the stable development, transfer and cleaning characteristics and environmental charging stability have not been obtained yet under the present conditions, and the image stability for a long period has been required. It has been therefore desired to provide full color toner images stable in image quality to environmental fluctuations and long-term use.